1. Technical Field
The present disclosure relates to systems for providing energy to biological tissue and, more particularly, to improved apparatus for amplifying energy for use during electrosurgical procedures.
2. Background of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, thermal, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator.
Ablation is most commonly a monopolar procedure that is particularly useful in the field of cancer treatment, where one or more RF ablation needle electrodes (usually having elongated cylindrical geometry) are inserted into a living body and placed in the tumor region of an affected organ. A typical form of such needle electrodes incorporates an insulated sheath from which an exposed (uninsulated) tip extends. When an RF energy is provided between the return electrode and the inserted ablation electrode, RF current flows from the needle electrode through the body. Typically, the current density is very high near the tip of the needle electrode, which tends to heat and destroy surrounding issue.
In bipolar electrosurgery, one of the electrodes of the hand-held instrument functions as the active electrode and the other as the return electrode. The return electrode is placed in close proximity to the active electrode such that an electrical circuit is formed between the two electrodes (e.g., electrosurgical forceps). In this manner, the applied electrical current is limited to the body tissue positioned immediately adjacent the electrodes. When the electrodes are sufficiently separated from one another, the electrical circuit is open and thus inadvertent contact with body tissue with either of the separated electrodes does not cause current to flow.
Commonly used power amplifiers are known to be inefficient. For example, a class A/B microwave power amplifier typically exhibits an efficiency of about 35%. That is, to achieve a surgical signal of 250 W, a class A/B power amplifier requires about 714 W of power, of which 464 W is dissipated as thermal energy. The resulting heat becomes difficult to manage and may require the use of bulky and costly cooling systems, e.g., fans and heat sinks. Additionally, the excess heat may cause thermal stress to other components of the generator, shortening generator life, decreasing reliability, and increasing maintenance costs. Additionally, a class A/B amplifier may exhibit crossover distortion that introduces undesirable harmonics into the surgical signal, which are known to cause radiofrequency interference in excess of acceptable limits.
Because electrosurgery requires very large ranges of voltage and current while maintaining a very high conversion efficiency to reduce the size and heat sink needs, some electrosurgical devices use a tuned resonance circuit driven by a class C or class E output. However, such an arrangement has a disadvantage in that the quality factor or Q factor changes with the load applied to the circuit making it difficult to maintain the desired voltage or current at the output. A manifestation of this disadvantage may be excessive or undesired “ringing” at the output for underdamped or high Q situations of loading. Class A/B outputs do not suffer from this disadvantage because their output impedances are typically much lower.